The output of a typical diesel engine mounted on, for example, a vehicle, is controlled by adjusting the amount of fuel injection. Therefore, the throttle opening degree is not finely controlled. During recent years, diesel engines with an exhaust gas recirculation device, or an EGR device have been developed. When using an EGR device, the amount of exhaust gas that is recirculated with intake air, or the EGR amount, needs to be finely controlled. For example, Japanese Laid-Open Patent Publication No. 2002-327643 discloses a diesel engine with an electronically controlled throttle valve that is capable of finely control the throttle opening degree in accordance with the operating state of the engine.
When a diesel engine is operated under a low atmospheric pressure condition, for example, at high altitude, the amount of new air that is drawn from the outside is reduced. Therefore, under a low atmospheric pressure, the reduction in the intake amount of new air drawn from the outside needs to be compensated for. Particularly, in the case of a diesel engine with an EGR device, reduction in the amount of intake air under a low atmospheric pressure increases the EGR ratio, or the ratio of the EGR amount in the total amount of gas that fills the cylinder. The combustion state deteriorates accordingly. Thus, Japanese Laid-Open Patent Publication No. 9-60559 discloses a diesel engine that corrects the throttle opening degree in accordance with the atmospheric pressure, that is, performs atmospheric pressure correction (high altitude correction) of the throttle opening degree, thereby compensating for reduction in the amount of intake air under a low atmospheric pressure.
Normally, the atmospheric pressure correction value of the throttle opening degree is obtained by referring to a one-dimensional computation map based on the atmospheric pressure shown in FIG. 7. The computation map stores appropriate atmospheric pressure correction values (A0, A1, . . . , An) that correspond to the values of the atmospheric pressure (P0, P1, . . . , Pn). A base value (base opening degree) of the throttle opening degree, which is computed based on the engine speed and the engine load, is corrected by using an atmospheric pressure correction value obtained from the computation map, so that a final throttle opening degree is set.
Through such an atmospheric pressure correction of the throttle opening degree, the reduction of intake air under a low atmospheric pressure is compensated for. However, if the correction of the throttle opening degree in accordance with the atmospheric pressure is executed in a uniform method for all the engine operating conditions, the following drawbacks occur.
The emission controllability of the EGR at high altitude deteriorates. Depending on the engine operating condition, the EGR ratio is set to a value near limitation beyond which deterioration of the combustion state cannot be avoided. Under such an engine operating condition, if the EGR ratio is increased by reduction in the amount of intake air due to a low atmospheric pressure, the combustion state immediately deteriorates. Therefore, the above described atmospheric pressure correction of the throttle opening degree needs to be performed such that the combustion state does not deteriorate even under an engine operating condition where the EGR ratio has little margin. On the other hand, depending on the engine operating condition, the EGR ratio is set to a value that does not cause the combustion state to deteriorate even if the EGR ratio is increased by a certain degree under the normal atmospheric pressure. In such an engine operation condition, where the EGR ratio has a sufficient margin, if the atmospheric pressure correction of the throttle opening is performed on the assumption that the engine operating condition has little margin, the correction will be excessive. Thus, the improvement of the emission performance by the EGR will be limited.
Also, if the atmospheric pressure correction of the throttle opening degree is executed in a uniform manner for all the engine operating conditions, the controllability of the exhaust temperature at high altitude is likely to deteriorate. Depending on the engine operation condition, the discharge amount of substances such as HC under the normal atmospheric pressure can be close to an allowable limit. Under such an engine operation condition, if the exhaust temperature increases due to a reduction in intake air under a low atmospheric pressure, the discharge amount of substances such as HC immediately surpasses the allowable limit. Therefore, the above described atmospheric pressure correction of the throttle opening degree needs to be performed such that the discharge amount of substances such as HC does not surpass the allowable limit even when the discharge amount has little margin. On the other hand, depending on the engine operating condition, the discharge amount of substances such as HC under the normal atmospheric pressure is sufficiently small. Thus, even if the discharge amount of such substances is increased due to a reduction in the intake air under a low atmospheric pressure, the discharge amount of such substances can be kept within an allowable range. Under an engine operating condition where the discharge amount of substances such as HC has a sufficient margin, if the atmospheric pressure correction of the throttle opening is performed on the assumption that the discharge amount of substances such as HC has little margin, the amount of intake air is excessively increased, so that the exhaust temperature is excessively lowered. This may hamper the temperature increase control of an exhaust purification catalyst.
Also, if the atmospheric pressure correction of the throttle opening degree is executed in a uniform manner for all the engine operating conditions, the controllability of the air-fuel ratio at high altitude is likely to deteriorate. In a diesel engine with an exhaust purification catalyst of NOx storage reduction type, sulfur component in exhaust gas is gradually accumulated in the exhaust purification catalyst, and the purification performance of NOx component is degraded. That is, sulfur poisoning occurs. Therefore, in such a diesel engine, the sulfur release control needs to be carried out in which the air-fuel ratio of air-fuel mixture is temporarily made rich according to the progress of sulfur poisoning, so as to remove sulfur component accumulated in the exhaust purification catalyst. On the other hand, the air-fuel ratio of air-fuel mixture to be burned varies according to the engine operating condition even under the normal atmospheric pressure. Depending on the engine operating condition, the air-fuel ratio under the normal atmospheric pressure is set to a value close to the limit on the rich side in an air-fuel ratio range allowing satisfactory combustion to continue. If the air-fuel ratio is made rich by reduction in intake air amount under a low atmospheric pressure, the combustion state may immediately deteriorates. Under such an engine operating condition, the above described atmospheric pressure correction of throttle opening degree needs to be performed so as to maintain the air-fuel ratio in the range that allows satisfactory combustion state to continue. On the other hand, in some cases, the diesel engine is operated at a relatively lean air-fuel ratio under the normal atmospheric pressure, depending on the engine operating condition. In such an engine operation condition, which has a sufficient margin for the air-fuel ratio to be made richer, if the atmospheric pressure correction of the throttle opening is performed on the assumption that the engine operating condition has little margin for making the air-fuel ratio richer, the intake air amount will be excessive, and the air-fuel ratio will be excessively lean. Even if the sulfur release control is executed in such a state, the air-fuel ratio cannot be made sufficiently rich, and the catalyst cannot be recovered from the sulfur poisoning.
As described above, if the atmospheric pressure correction of the throttle opening degree is executed in a uniform manner for all the engine operating conditions, the controllability of various parameters will be degraded as shown above. Such problems can be avoided by computing the atmospheric correction value by taking into consideration not only the atmospheric pressure, but also other parameter such as the charging pressure. However, in such a case, the computation of the atmospheric pressure correction value needs to be executed using two- or more dimensional map. Creation of such a map requires a great number of adaption steps. The capacity of such a map is inevitably increased and thus reduces the open storage space.